Hi Alena,
I would suggest you purchase or borrow a GPS unit (handheld in a waterproof pack/bag) and do your own tests.
Try your 10.0 m2 rig ....its heavy and cumbersome to use such a large sail on such a narrow platform as the Serenity.
Then take a 7.5 m2 free race sail (Sailworks Retro etc) and sail about the same courses.
I think you will find the 10.0 m2 rig that JM uses is to race with sailboats on a regular yacht racing triangular
course, not free sailing back and forth.
I will be very suprised if you find that the 10.0 m2 moves the Serenity more that 1 knot faster than
a good drafty 7.5 m2 rig in 5-10 knots of wind.

I sailed the Mk 1 Serenity with sails as small as a 4.2 m2 Sailworks Retro Ripper in fairly light winds and it moved along and tacked/jibed just fine.
I spent alot of time with Jim Drake when he came down to Hatteras as the designer of the Serenity he placed effciency at the top of the list in his design criteria.
His recommended sail size was 7.5 m2 as the optimum.
I've had the Serenity planing (it planes and moves very fast until the tail goes under then it gets real
unstable) with a 4.8 m2 Sailworks Hucker in 20 knots of wind.
As far as tacking, learn to put the mast back as far as you can and keep it there with the foot of the sail
right down on the deck of the Serenity) and learn to step over the mast so you do not bring the sail back up until the nose of the board has passed through the eye of the wind.
Serenity's do not tack real fast, but if you use all the forces and alignments at your disposal, you can get it around fairly quickly. I does take some practice.
What fin are you using?
I often found that a smaller fin (60 cm slalom or race vertical fins worked the best) helped with both the boardspeed (less drag) and made the Serenity significantly easier to turn, tack, and jibe.
You might loose a tiny bit of upwind capability, but the overall Serenity experience was much better with a 60 cm fin.
Per JIm Drake, at your weight, put the mast foot as far forward as it will go.
Go upwind by tipping the board slightly toward the lee rail as this really improves the way the nose bites
into the water and increases the upwind capabily of the fin.
Hope this helps,
Roger